We have engineered a null mutation (knockout) in the MMP-13 gene that encodes a collagenase predominantly expressed in the Skeleton. MMP-13 null (-/-) mice have a striking skeletal phenotype with abnormal growth plates and delayed ossification during embryonic development and, after birth, increased deposition of trabecular bone due to decreased osteoclast and increased osteoblast function. We had previously targeted a mutation to Col1a1 (r/r mice) that that results in complete resistance to collagenase cleavage in helical domain of type I collagen. The phenotype of the r/r mice differs in several respects from that of the MMP-13 null mice. Our goals here are to define the mechanisms for these biological effects. We will evaluate skeletal cell function in these models in vivo and ex vivo and explore related animal models. Specific Aim 1: Investigate mechanisms that underlie the abnormal skeletal phenotype of the MMP-13 -/- (knockout) mice using in vivo and ex vivo. In vivo, we will further explore the phenotype of the MMP-13 -/- mice using procedures that perturb skeletal remodeling. Ex vivo, we will examine osteoblast and osteoclast differentiation and function pertinent to understanding the phenotype in the MMP-13 -/- mice. Specific Aim 2: Investigate mechanisms that underlie the abnormal skeletal and extraskeletal phenotype of the collagenase-resistant (r/r) mice. MMP-13 makes additional unique cleavages in the N-telopeptide of type I collagen. Here, we will cross the r/r mice with the MMP-13 -/- mice, analyze the in vivo phenotype and examine skeletal cell differentiation/function ex vivo. These studies will identify roles of MMP-13 other than proteolysis of helical type 1 collagen. Specific Aim 3: Determine the role of other MMPs in murine skeletal development and remodeling by examining other in vivo models. We will analyze the skeletal and extraskeletal phenotype of mice with a null mutation in the "neutrophil" collagenase, MMP-8, another collagenase expressed in the skeleton, as well as mice with double null MMP-8/MMP-13.